WO 96/35685 discloses an (RRR)-optical isomer of 1,4-benzodioxin-2-carboxylic acid derivatives represented by the following general formula (7') which is useful as a prophylactic and therapeutic agent for diabetes, hyperglycemia and the like, and a process for the preparation thereof. ##STR4##
wherein R.sub.1 is hydroxy or (C.sub.1 -C.sub.4)alkoxy, and R.sub.2 and R.sub.3 may be the same or different, and each is hydrogen, halogen, (C.sub.1 -C.sub.6)alkyl, trifluoromethyl, (C.sub.1 -C.sub.6)alkoxy, aryl, aryloxy or aryl(C.sub.1 -C.sub.6)alkyloxy, the aryl, aryloxy or aryl(C.sub.1 -C.sub.6)alkyloxy being optionally substituted by one or two halogens, or R.sub.2 and R.sub.3 may together form --OCH.sub.2 O--.
According to a method disclosed in WO 96/35685, a compound of the above formula (7') is prepared as shown in the following Scheme I. A compound of formula (6) is reductively condensed with a compound of formula (8) to form a mixture of diastereoisomers of formula (9), which is then converted into an N-tert-butoxy carbonyl derivative of formula (10), and the derivative is separated into diastereoisomers of formulae (11) and (12) by column chromatography, and then a compound of formula (11) is hydrolyzed to prepare an (RRR)-optical isomer of 1,4-benzodioxin-2-carboxylic acid derivatives represented by the formula (7'). ##STR5##
The above-mentioned process has not been accepted as an efficient method, because it forms a compound of formula (12) which is not required for the production of a desired compound [formula (7')], which resulted in reducing a total yield of the desired compound.
Therefore, it has been demanded to efficiently produce an (RRR)-optical isomer of 1,4-benzodioxin-2-carboxylic acid derivatives represented by the formula (7'), which is useful as a prophylactic and therapeutic agent for diabetes, hyperglycemia and the like.